Carriers that carry liquefied gas, such as liquefied natural gas (LNG) or liquefied petroleum gas (LPG), include a tank that accommodates the liquefied gas. If the liquefied gas within the tank is not in a supercooled state, the liquefied gas within the tank evaporates due to the heat that enters the inside of the tank from the outside of the tank. If the amount of evaporation of the liquefied gas exceeds the amount of the gas to be discharged from the tank, the pressure within the tank increases.
The tank includes a safety valve such that the pressure within the tank does not increase excessively. A pressure value at which the safety valve operates is set on the basis of the allowable pressure or the like of the tank. The safety valve opens a main valve inside the safety valve and releases the gas within the tank to the outside when the pressure within the tank reaches a predetermined operating pressure value.
During the navigation of the carriers, the amount of evaporation of the liquefied gas may increase due to the fluctuation of the liquefied gas within the tank in addition to the heat that enters the inside of the tank from the outside of the tank. Thus, a configuration in which the set pressure value of a safety valve is changed during navigation is disclosed in Patent Document 1.
Here, as the safety valve, there is a direct acting type or a pilot type. In the direct acting type safety valve, a valve body of a main valve is directly pressed against a valve seat from one side by a spring, and when the pressure acting on the other side of the valve body exceeds a pressing force generated by the spring, the main valve is opened. Meanwhile, the pilot type safety valve has a structure in which areas that receive pressures are made different while making a pressure acting on one side of the valve body and a pressure acting on the other side equal to each other, and a load that presses the valve body against the valve seat is exerted. In this pilot type safety valve, as a pilot valve reduces the pressure that presses the valve body against the valve seat, the valve body is separated from the valve seat and the main valve is opened. The structure of the pilot valve resembles the structure of the above-described direct acting type safety valve, the valve body of the pilot valve is directly pressed from one side by the spring, and when the pressure acting on the other side of the valve body exceeds the pressing force generated by the spring, the valve body is separated from the valve seat and the pilot valve is operated. Such a pilot type safety valve is opened and releases pressure when the pressure on the other side of the main valve acts on the pilot valve and exceeds the pressing force generated by the spring of the pilot valve. The pressure that presses the valve body of the pilot valve against the spring is equal to the pressure acting on the valve body of the main valve, and if the pilot valve operates, the pressure that presses the valve body of the main valve against the valve seat decreases, and the main valve is opened. Generally, the pilot type safety valve is used for tanks of large-sized liquefied gas carriers.
In order to switch the operating pressure value of such a pilot type safety valve, it is necessary to replace the spring itself of the pilot valve with one having a different spring constant or change the number of springs to be mounted on the pilot valve.